1. Field of the Invention
The present invention generally relates to electronic video devices, and more particularly to a method of decoding or separating video signal components (i.e., chrominance and luminance) in a composite video signal.
2. Description of the Related Art
Various electronic video devices such as televisions, video cassette recorders and digital video disc (DVD) players utilize a composite video signal to record, transmit, and reproduce video images. The composite video signal typically includes a luminance (intensity) component denoted “Y” and a chrominance (color) component denoted “C”. This type of video signal encoding is common to two well-known video standards, the National Television System Committee (NTSC) standard used in North America and Japan, and the Phase Alternating Line (PAL) standard used in most of Europe, Africa and Asia. The chrominance component of the signal is encoded on a subcarrier and added to the luminance signal. For NTSC, the chrominance is modulated on a subcarrier frequency of 3.579545 megahertz (MHz), and for PAL the chrominance is modulated on a subcarrier frequency of 4.433619 MHz, using quadrature modulation.
When the transmitted composite video signal is received at a video device, the luminance and chrominance components need to be separated out in order to determine their respective values and effectuate the color scheme. Many different video decoder designs have been devised for this purpose. Some of these decoders use one or more line delays and adders to cancel out the luminance component, and a bandpass filter to obtain the chrominance component. A conventional two-line delay YC separator circuit 10 for general viewing of a composite NTSC video signal is depicted in FIG. 1.
The composite video signal input of separator circuit 10 is fed to a first line delay 12 and an adder 14. The output of first line delay 12 passes to a second line delay 16 and two invert adders 18 and 20. The output of second line delay 16 passes to the other input of adder 14. Adder 14 generates a double-amplitude composite video signal since the subcarriers are in-phase. A divider 22 (i.e., 0.5 multiplier) is used to normalize the signal, which is then fed to the negative input of invert adder 18. Since a 180° phase difference exists between the output of adder 14 and the one line-delayed composite video signal, most of the luminance is canceled by adder 18, leaving double-amplitude chrominance. Another divider 24 is used to normalize the chrominance signal, which is then fed to a bandpass filter 26. The output of bandpass filter 26 is the chrominance output signal. This signal is also fed to the negative input of adder 20 to yield the luminance output signal.
Many composite video signal decoders use comb filters. Comb filters combine a scan line with one or more previous scan lines (scan lines are a horizontal line as displayed on a monitor or screen). Under the NTSC format, the chrominance phase changes 180° from one scan line to the next scan line. As a result, if two adjacent scan lines are identical, then adding them will eliminate the chrominance component, leaving only luminance. The same concept can be applied to PAL signals by using four line delays.
Several disadvantages to line-delay circuits, such as that shown in FIG. 1 exist. A principal disadvantage is the unsuppressed cross-luminance on vertical color transitions, i.e., incorrect decoding of the luminance value due to abrupt color changes. Circuits that use comb filters can have further problems with diagonal lines and vertical color changes since only vertically-aligned samples are processed. Also, with diagonal lines, after standard comb filtering, the chrominance information includes the difference between adjacent luminance values, which may be misinterpreted as chrominance information. This difference can appear as cross-color artifacts along the edge of the line. Sharp vertical color transitions can further generate the “hanging dot” pattern commonly seen on the scan line between the two color changes.
Several different adaptive decoders have been designed which attempt to resolve these problems in Y-C separation. For example, U.S. Pat. No. 6,462,790 discloses a digital comb filter for decoding composite video signals which uses a fast Fourier transform circuit or band split filter circuit to determine characteristics of the input video signal without demodulating the signal. Those circuits produce signature signals, which are then used to correlate each of the video lines and compute weighting coefficients for a comb filter. The device uses the sum of four surrounding lines that are out of phase with the current line for comb filtering. The comb filter reverts to a band split filter if none of the surrounding lines are similar. The weighting coefficients can be adjusted as the noise level increases or decreases.
U.S. Patent Application Pub. No. 2002/0140866 discusses an adaptive comb filter design for separating chrominance and luminance components, which provides a threshold determination of whether the lines of chrominance are correlated, and uses a comb filter to separate out the Y-C information. If adjacent scan lines of chrominance contain the same color information or differ by only a slight level, the scan lines are considered correlated. If the lines are uncorrelated, then a bandpass filter is used to separate out the chrominance component from the composite video signal.
U.S. Patent Application Pub. No. 2002/0149702 describes another decoder for composite video signals, which prevents decreases in resolution at the time of Y-C separation associated with images having vertical stripes. A correlation judging section determines whether there is any correlation among neighboring signal lines, and one of two filters (either a two-line comb filter or a three-line comb filter) is selectively used depending on the correlation. A stripe component judging section examines neighboring pixels on the signal to indicate the presence of a stripe.
U.S. Patent Application Pub. No. 2003/0071921 teaches a luminance-chrominance signal separation device, which detects diagonal components of the luminance signal and then utilizes different bandpass filters to separate out the chrominance signal based on the amount of diagonal components in the luminance signal. Use of two different bandpass filters (a broad-bandpass filter and a narrow-bandpass filter) reduces cross-color in the output chrominance signal and improves resolution in the diagonal direction of the output luminance signal.
While each of these designs has certain advantages, they all still suffer from various limitations that are not addressed simply by the selective use of bandpass filters versus comb filters. It would, therefore, be desirable to devise an improved decoder for separating out luminance and chrominance values in a composite video signal, which more comprehensively addresses issues of vertical luminance resolution, cross-chrominance artifacts, and chrominance transitions. It would be further advantageous if the decoder could retain high quality transmission of signals with no chrominance content.